As will be appreciated in man portable targeting systems which contain reticles usually mounted on weapons, there is a requirement to be able to accurately target an individual or piece of equipment to within milliradian accuracy. This requirement necessitates an accurate north reference so that the bearing to the target can be accurately ascertained.
Keeping north with conventional inertial measuring units (IMUs) is not currently feasible with man portable military systems. The IMUs necessary for any persistent north keeping in excess of five minutes have weights of more than half pound, consume 3-5 watts (W) of power and have accuracy issues dependent on temperature. Therefore, there is a requirement for an improved way of establishing a north reference in an IMU.
Heretofore, the problem with providing an IMU with a north reference is that either a compass or another mechanical device or hardware was required to find the north reference. While it might be presumed that one could provide a man portable unit with an inertial reference device, such as a north finding gyroscope/compass, the fact is inertial reference devices are exceedingly heavy to ensure durability or ruggedized enough to be used in the field. Additionally, the high cost of such inertial reference devices as well as their power consumption limit the ability to use them on man portable units.
The problem with utilizing a magnetic compass is that the north keeping capability of the compass is constantly varying due to magnetic influences surrounding the compass. This will alter the compass-derived north vector making the compass relatively useless in providing milliradian accuracies.
As to the use of north finding gyroscopes associated with inertial reference devices, these devices require 3-4 minutes of integration time in a non-motion environment, and require a large amount of maintenance, all of which are difficult in the field. Moreover, these devices weigh 3-5 pounds or more due to their inertial gyroscopes, are unwieldy due to their weight alone, and are too bulky to be used in man portable equipment. Additionally, power consumption of between 3 and 5 watts requires large, heavy batteries.
While there exist high resolution maps and while there are compasses and north finding gyros, at present there is no complete man portable targeting system due to the weight, power requirements and time limits for finding north.
Further, there are some celestial devices that can find north extremely accurately. However, on a cloudy day stars can be occluded or will not be available for brief periods of time. Thus, trying to utilize real-time celestial navigation devices for targeting is problematic. Even if true north can be ascertained and maintained to milliradian accuracy, this accuracy is only available for 20 to 30 seconds and degrades over time. After about three minutes the north keeping capability degrades so badly that one cannot target with it any longer.
In an effort to provide military personnel with the ability to provide targeting information for precision ordinances, in the past, a targeting system would survey an engagement area such that there is a reticle placed on a potential target. The scene available to the targeting system does not have an accurate north reference unless provided by an IMU. If within the engagement area there is a north feature such as a building corner, the targeting system's reticle can be manually shifted to overlie the north feature so that the azimuth of the target from north can be ascertained. This overlying procedure requires consummate skill on the part of the soldier and the accuracy is questionable during battlefield conditions due to the inability of the soldier to align his reticle with the corresponding north feature.
Thus, there is a necessity to eliminate manual correction of the north reference supplied to an IMU to eliminate the requirement of manual skill There is therefore a need for an easy to use automatic north reference keeping capability for the IMUs used in targeting.